1. Field of the Invention
The present invention relates to treatment of webs and films of fibrous material more particularly, to wire corona charging apparatus and processes.
2. Description of Background Art
Corona processes are used for surface treatment of fiber webs and films. Although surface treatment can have many other objectives, the most important or common objectives are to increase wettability for printing, to increase absorptive characteristics (see, for example, Dinter et al., U.S. Pat. No. 5,135,724), and to produce permanently charged materials that are typically referred to as electrets (see, for example, Wadsworth and Hersh, U.S. Pat. No. 4,375,718). Such processes involve apparatus for causing corona discharge. Corona producing apparatus that can be used in these surface treatment processes are commonly referred to as "ionizers", "corona treaters" or "corona devices" or "chargers".
Thin wires produce a highly controlled and well distributed ion flux, when compared to more commonly used corona devices such as charge bars, rods, and needle points, principally because of the small diameter of the wires. Corona streamers produced from smaller diameter wires are more uniform across the treatment surface than those produced by using larger diameter rods or sharp edges or needle points (White, Industrial Electrostatic Precipitation, Addison-Wesley Publishing Company, Inc. 1963). Additionally, the amount of ozone, a pollutant, produced by small wire corona chargers is lower than the amount of ozone produced by larger diameter wires, rods, etc. (Whit, 1963). Another advantage of wire corona chargers is that due to the highly distributed ion flux, (i.e., uniform streamers) there is a lower possibility of producing violent, high density sparks that can cause pitting in counter potential rollers (see, for example, Schuster U.S. Pat. No. 4,281,247) that are coated with a dielectric layer (typically a ceramic coating). Corona produced from other devices, such as charge bars, rods, or sharp points results in such sparks and pitting under many conditions. Such rollers are expensive, and thus pitting can substantially increase operating cost when such corona charge bars are used.
Although wires are commonly suggested as options for use in corona charging equipment, we have found that wires are seldom, if at all, used in corona devices for treatment of webs or films that are over 24-30 inches wide because wires require high axial tension when strung over wide widths, in order to prevent slack from occurring in the middle of the wires. In contrast, bars, rods and sharp points do not require axial tension for mounting in wide ionizers. By way of explanation, rods and bars, regardless of their specific cross-sectional shape (with that cross-sectional shape taken within a plane dividing the rod or bar and defining an orthogonal angle with the longitudinal axis of the rod or bar) are elongate elements having sufficiently large dimensions within that plane that the linear measurement of deflection of the centroid of a bar or rod over a span where the unloaded bar or rod is simply supported only at its opposite ends is significantly less than the greatest value of a cross-sectional dimension of the bar or rod taken along a line parallel to the path of the centroid during the deflection. In contradistinction, although a wire is also an elongate element, the centroid if an unloaded wire simply supported only at its opposite ends will freely trace a path during deflection of the wire that is many times greater than the greatest cross-sectional dimension of the wire taken along a line parallel to that path, even while the opposite ends of the wire are held under tension. Consequently, to restrict the deflection of a centroid of a wire to a value that is comparable to that of a bar or rod of the same length, it is necessary to hold the opposite ends of the wire with such a high degree of tension that substantial risk exists that the wire will break. Wires used in wide corona chargers can therefore easily break and cause safety hazards in these surface treatment processes, which are typically continuous processes running at high speeds. We have also found that wires can get snagged with the film or roll moving at high speed, thus causing safety problems, and damage to process components. In order to alleviate this breakage problem, tungsten wires have been suggested, chiefly due to the high tensile strength of tungsten. Typically, tungsten wires with 0.2-1 millimeter diameters are preferred for corona charging processes (cf. Nakao, U.S. Pat No. 4,582,815). Many web and film processes however involve web or film widths between 50-120 inches. Over such lengths, even tungsten wires of these small diameters can break while under tension.
We have noticed that another reason for the lack of use of wires in contemporary wide corona chargers is that over time, the wire can relax and, when the wire lengths are long, the slack in the middle part of the wire can produce field strength variations and, in some cases, may even become tangled with the web or film.
Primarily due to the safety issues due to breakage of wires, typically charge bars (as is suggested by Wadswo and Hersh, U.S. Pat. Nos. 4,375,718 and by Dinter et al., 5,135,724) are preferred in such applications, even though corona produced by thin wires have distinct advantages that are not available with charge bars. Additionally, charge bars are preferred in contemporary chargers because many charge bar designs enable the introduction of gases or aerosols (as, for example, Dinter et at., 5,135,724 and Kubik and Davis, 4,215,682) into the corona- these gases or aerosols are thought to enable better or specific surface treatment of the fibrous materials, often through surface chemical reactions induced by corona treatment. Currently, there are no wire based chargers on the market that facilitate the introduction of aerosols and or gases into the corona region.